Self light emitting device

ABSTRACT

To provide a method of improving an efficiency for extracting light in a self light-emitting device using an organic EL material. In the self light-emitting device having a structure in which an EL layer ( 102 ) is sandwiched between a transparent electrode ( 103 ) and a cathode ( 101 ), a film thickness of the EL layer ( 102 ) and a film thickness of the transparent electrode ( 102 ) are equivalent to the film thicknesses in which there is no occurrence of a guided light, and an inert gas is filled in a space between the transparent electrode ( 103 ) and a cover material ( 105 ).

This application is a continuation of U.S. application Ser. No.11/021,070, filed on Dec. 22, 2004 now U.S. Pat. No. 7,317,282 which isa continuation of U.S. application Ser. No. 09/696,619, filed on Oct.25, 2000 (now U.S. Pat. No. 6,847,163 issued Jan. 25, 2005).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a self light-emitting device includingan EL element, which is formed on a substrate, and comprises an opaqueelectrode (cathode), a transparent electrode (anode), and a luminescingorganic material (hereinafter referred to as an organic EL material)sandwiched therebetween. Specifically, the present invention relates toan improvement of efficiency for extracting light from the EL element.

2. Description of the Related Art

In recent years, developments of EL display devices employing as anelement an organic EL material are advancing. This is a current drivingtype self light-emitting device utilizing light emitting, which iscaused by re-coupling of an electron and a hole injected into an organicthin film from the electrodes of both surfaces by applying a voltage.The light emitted is extracted as a sheet luminescent. However, theefficiency of extracting light, generated in a solid-state thin filmhaving a large refractive index, to outside of the light-emittingelement as a sheet luminescent is extremely low, normally 20% or less.

As shown in FIG. 2, among the lights outputted from a light source “A”in an “a” layer 202 having a large refractive index (n=n₁) to “b” layers201 and 203 having small refractive indexes (n=n₂), the light enteringat angles (θ₁ and θ₂) greater than the radiation angle of θ₀ (providedthat θ₀=sin⁻¹(n₂/n₁)) is entirely reflected and is waveguided in the “a”layer with a large refractive index. Thus, light that is waveguided isreferred as a guided light. A part of the components of the guided lightlight is absorbed and disappears while the rest is propagated in the “a”layer 202 and escapes to the edge surface. Therefore, only a portion ofthe outgoing light can be extracted as a sheet luminescent.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above stated problem,and therefore it is an object of the present invention to improve theefficiency of extracting light in a luminescent element, particularly,an EL element. Further, another object of the present invention is toprovide a self light-emitting device having a high luminescingefficiency.

In the structure of the self-light emitting device of the presentinvention, as shown in FIG. 1, each of an EL layer 102 and a transparentelectrode 103 contained in an EL element is formed so as to have athickness in which a guided light does not occur, and an inert gas isfilled between the transparent electrode and a cover material 105 (aregion denoted by the reference numeral 104). Note that the regiondenoted by the reference numeral 104 is referred to as a “gas space”throughout the present specification.

Throughout the present specification, an EL element is an element whosestructure is composed of a cathode made of an opaque electrode, an anodemade of a transparent electrode, and an EL layer sandwichedtherebetween. In the present invention, the cathode is the opaqueelectrode (light shielding electrode) and the anode is the transparentelectrode. Further, it is to be noted that a layer where the injection,transportation, and re-coupling of carriers are performed is referred toas the EL layer throughout the present specification.

Furthermore, the film thickness in which there is no occurrence of aguided light is referred to as a film thickness (d) derived from d≦λ/(4n), where the refractive index of the film is n and the wavelength ofthe light generated in the EL element is λ. For instance, if thewavelength of the light generated in the EL element is 560 nm and therefractive index of the film is n_(x), then d≦(140/n_(x)). Therefore,there is no existence of a guided light in a film having a filmthickness that is thinner than the film thickness (d). Note that it isappropriate to set the total film thickness to (140/n_(x)) or less whenthe refractive indexes of both the film for constituting the EL layerand the transparent electrode are n_(x).

Next, an explanation will be made regarding the structure in which aninert gas is filled into the space between the transparent electrode andthe cover material. It is generally known that when light travelsthrough a gas space, a solid-state layer, and a gas space again in thatorder, the light can be extracted efficiently. Therefore, the structureis formed such that light generated in the EL element travels throughthe gas space, the solid-state layer, and the gas space in that orderafter it has transmitted the transparent electrode, thereby making itpossible to improve the efficiency of extracting light. In the presentinvention, the structure of the EL element is a structure in which theinert gas is sandwiched between the transparent electrode and the covermaterial, and thus by forming the above stated structure, light can beextracted efficiently.

When employing the EL element having the structure shown in FIG. 1, itis desirable to provide a buffer layer between the EL layer 102comprising only a light-emitting layer and the transparent electrode103, or between the EL layer 102 made of only a light-emitting layer andthe opaque electrode. It is to be noted that the buffer layer indicatesa layer for promoting the injection and transportation of carriers(electrons or holes). In other words, an electron injection layer, ahole injection layer, an electron transporting layer, or a holetransporting layer may be utilized as the buffer layer. In the case ofproviding a buffer layer, the buffer layer is contained in the EL layerthroughout the present specification.

The provision of the buffer layer improves the state of interfacebetween the light-emitting layer and the electrode, and therefore theefficiency of extracting light generated in the light-emitting layer isimproved. In the case of providing a buffer layer in the EL element withthe structure of FIG. 1 of the present invention, the luminescingefficiency is further enhanced. Moreover, a problem of damaging thelight-emitting layer during forming the transparent electrode thereoncan be resolved by forming the buffer layer sandwiched between thelight-emitting layer and the transparent electrode.

Thus, in the present invention, the film thicknesses of the respectivelayers forming the EL layer 102 and the film thickness of thetransparent electrode, which are included in the EL element, are filmthicknesses in which there is no occurrence of a guided light, and thereis inert gas in the space between the transparent electrode and thecover material 105, thereby making it possible to largely increase theefficiency of extracting light. In addition, by providing the bufferlayers between the electrodes that sandwich the light-emitting layer andthe light-emitting layer within the EL element to form the EL layer madeof the light-emitting layer and the buffer layers, the efficiency ofextracting light can further be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram showing a structure of a self light-emitting deviceof the present invention;

FIG. 2 is a diagram showing the state of generating a guided light;

FIG. 3 is a diagram showing a structure of a self light-emitting deviceof the present invention;

FIG. 4 is a diagram showing a structure of a self light-emitting deviceof the present invention; and

FIG. 5 is a diagram showing a structure of a self light-emitting deviceof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the present invention, as shown in FIG. 1, an EL element has astructure wherein an EL layer 102 and a transparent electrode 103contained therein is formed so as to have a film thickness where thereis no occurrence of a guided light, respectively, and a gas space 104 isformed sandwiched between the transparent electrode 103 and a covermaterial 105, and spacers 106 are utilized in the gas space 104 tosecure it. To give an actual example, the film thickness of the EL layer102 is set to 30 nm and the film thickness of the transparent electrode103 is set to 100 nm.

A polymer-based organic EL material or a monomer-based organic ELmaterial is used for the EL layer 102. The polymer-based organic ELmaterial can be dissolved in a solvent in the polymer state and thenapplied, or it can be dissolved in a solvent in the monomer state andthen polymerized after it has been applied.

Note that the gas space 104 refers to the space filled with the inertgas (typically argon, nitrogen, neon, and krypton). The cover material105 indicates a transparent member, and specifically, glass, quartz,plastic, etc. can be used.

Further, in order to improve the interface state in the transparentelectrode and the EL layer comprising only a light-emitting layer, asshown in FIG. 3, the structure has buffer layers 302 and 304 sandwichedbetween a light-emitting layer 303 and a transparent electrode 306, andbetween the light-emitting layer 303 and an opaque electrode 301,thereby forming an EL layer 305 comprising the light-emitting layer 303and the buffer layers 302 and 304. Note that a gas space 307, spacers308, and a cover material 309 are similar to the gas space 104, thespacers 106, and the cover material 105 in FIG. 1, respectively.

Embodiment 1

FIG. 4 is a diagram showing the cross sectional structure of an activematrix type self light-emitting device of the present invention. In FIG.4, reference numeral 401 denotes a substrate and 402 denotes a TFT. Itis to be noted that a known TFT is used as the TFT 402. Furthermore,reference numeral 403 denotes an electrode having aluminum (AL) as itsprincipal constituent, and a polyparaphenylene vinylene (PPV) is usedfor an EL layer 404. Reference numeral 405 denotes a transparentelectrode made of ITO, and argon is filled in a gas space denoted by406. Also, glass is used as a cover material 407 and spacers 408 areutilized in the gas space 406 to secure it.

Embodiment 2

FIG. 5 is a diagram showing a cross sectional structure of a passivematrix type self light-emitting device of the present invention. In FIG.5, reference numeral 501 denotes a substrate and 502 denotes an ELlayer. PPV is used for the EL layer. Reference numeral 503 denotes aplurality of opaque electrodes (cathodes) arranged in stripe shapes anda plurality of transparent electrodes (anodes) 504 are provided instripe shapes so as to be orthogonal to the plurality of opaqueelectrodes 503.

In addition, the EL layer 502 is formed sandwiched between the pluralityof opaque electrodes 503 and the plurality of transparent electrodes504. At this point, a cover material 506 made of glass is provided onthe plurality of transparent electrodes 504, sandwiching spacers 507. Agas space 505 is thus formed between the cover material 506 and theplurality of electrodes 504. Nitrogen is filled in the gas space 505 inEmbodiment 2. Note that the constitution of Embodiment 2 may beimplemented by freely combining it with the constitution of Embodiment1.

Embodiment 3

As shown in FIG. 3, the EL layer 305 comprises the light-emitting layer303 and, as buffer layers, the electron injection layer 302 and the holeinjection layer 304. A polymer-based material is used for the layersconstituting the El layer 305. For example, polyparaphenylene vinylenecan be used for the light-emitting layer 303, copper pthalocyanine orPEDOT for the buffer layer (hole injection layer) 304, and lithiumfluoride or lithium for the buffer layer (electron injection layer) 302.

Note that in forming the EL layer 305, it is desirable that thetreatment atmosphere for conducting the formation of this layer is a dryatmosphere having as little moisture as possible and in inert gas. Theinert gas here is one such as nitrogen or argon. Because the EL layereasily deteriorates due to the presence of moisture or oxygen, it istherefore necessary to eliminate these factors as much as possible whenforming this layer. In addition, these layers which constitute the ELlayer 305 may be provided commonly for all the pixels, and thereforethey may be formed by employing the spin coating method or the printingmethod.

The electron injection layer 302, which is the buffer layer, has a roleto inject electrons from the cathode 301 to the light-emitting layer303, and the hole injection layer 304 has a role to inject holes fromthe transparent electrode (anode) 306 to the light-emitting layer 303.In addition, by providing the hole injection layer 304, the preventionof damages to the light-emitting layer 303 during the formation of theelectrodes can be expected. Note that the constitution of Embodiment 3may be implemented by freely combining it with the constitution ofEmbodiments 1 or 2.

Embodiment 4

In manufacturing the EL element, a tris (8-quinolinolate) aluminumcomplex (Alq₃) is used for the light-emitting layer, and when usingmagnesium and silver (MgAg) for the cathode, the evaporated complexes ofboth the Alq₃ and the acetylacet sodium complexes can be used as thebuffer layer sandwiched between the light-emitting layer and thecathode. Note that the constitution of Embodiment 4 may be implementedby freely combining it with any of the constitutions of Embodiments 1 to3.

Embodiment 5

A self light-emitting device having a structure in which an organic ELmaterial is used as a light-emitting layer sandwiched between a cathodeand an anode has excellent brightness as well as low power consumption,and hence may be utilized as a back light of a liquid crystal displaydevice, etc. Regarding the cathode, the anode, and the light-emittinglayer, each can be formed on the entire surface of the substrateprovided that the cathode and the anode are formed in a range that theydo not directly contact each other. Furthermore, materials such as PPVand PVK (polyvinyl carabazole) may be used as the organic EL material.The present invention may be employed as the back light of a liquidcrystal display utilized in the display portion of a mobile phone, themonitor of a personal computer (PC), and the like. Note that theconstitution of Embodiment 5 may be implemented by freely combining itwith any of the constitutions of Embodiments 1 to 4.

1. An EL element comprising: an opaque electrode over a substrate; an ELlayer including a first buffer layer over the opaque electrode, alight-emitting layer over the first buffer layer, and a second bufferlayer over the light-emitting layer; a transparent electrode over the ELlayer; a cover material over the transparent electrode; an inert gasfilled in a space between the transparent electrode and the covermaterial; and spacers each having an oval or circular cross-section inthe space and provided on the transparent electrode, wherein the covermaterial is in contact with the spacers, and wherein each of the firstbuffer layer and the second buffer layer comprises one of copperpthalocyanine, PEDOT, lithium fluoride and lithium.
 2. The EL elementaccording to claim 1, wherein the cover material includes plastic. 3.The EL element according to claim 1, wherein the opaque electrodeincludes aluminum as its principal constituent.
 4. A light-emittingdevice comprising an EL element comprising: an opaque electrode over asubstrate; an EL layer including a first buffer layer over the opaqueelectrode, a light-emitting layer over the first buffer layer, and asecond buffer layer over the light-emitting layer; a transparentelectrode over the EL layer; a cover material over the transparentelectrode; an inert gas filled in a space between the transparentelectrode and the cover material; and spacers each having an oval orcircular cross-section in the space and provided on the transparentelectrode, wherein the light-emitting device further comprises a TFTover the substrate and the EL element is electrically connected to theTFT, wherein the cover material is in contact with the spacers, andwherein each of the first buffer layer and the second buffer layercomprises one of copper pthalocyanine, PEDOT, lithium fluoride andlithium.
 5. The light-emitting device according to claim 4, wherein thecover material includes plastic.
 6. The light-emitting device accordingto claim 4, wherein the opaque electrode includes aluminum as itsprincipal constituent.
 7. A light-emitting device comprising an ELelement comprising: a plurality of opaque electrodes arranged in stripeshapes over a substrate; an EL layer including a first buffer layer overthe plurality of opaque electrodes, a light-emitting layer over thefirst buffer layer, and a second buffer layer over the light-emittinglayer; a plurality of transparent electrodes arranged in stripe shapesso as to be orthogonal to the plurality of opaque electrodes over the ELlayer; a cover material over the plurality of transparent electrodes; aninert gas filled in a space between the plurality of transparentelectrodes and the cover material; and spacers each having an oval orcircular cross-section in the space and provided on the plurality oftransparent electrodes, wherein the cover material is in contact withthe spacers, and wherein each of the first buffer layer and the secondbuffer layer comprises one of copper pthalocyanine, PEDOT, lithiumfluoride and lithium.
 8. The light-emitting device according to claim 7,wherein the cover material includes plastic.
 9. The light-emittingdevice according to claim 7, wherein the plurality of opaque electrodesinclude aluminum as its principal constituent.
 10. A light-emittingdevice comprising: a TFT comprising: a semiconductor film over asubstrate; a gate insulating film over the semiconductor film; a gateelectrode over the gate insulating film; and an insulating film over thegate electrode, an EL element comprising: an opaque electrodeelectrically connected to the semiconductor film; an EL layer includinga first buffer layer over the opaque electrode, a light-emitting layerover the first buffer layer, and a second buffer layer over thelight-emitting layer; a transparent electrode over the EL layer; a covermaterial over the transparent electrode; an inert gas filled in a spacebetween the transparent electrode and the cover material; and spacerseach having an oval or circular cross-section in the space and providedon the transparent electrode, wherein the cover material is in contactwith the spacers, and wherein each of the first buffer layer and thesecond buffer layer comprises one of copper pthalocyanine, PEDOT,lithium fluoride and lithium.
 11. The light-emitting device according toclaim 10, wherein the cover material includes plastic.
 12. Thelight-emitting device according to claim 10, wherein the opaqueelectrode includes aluminum as its principal constituent.